Display screen or portion thereof with a graphical user interface



FIG. 1 is a front view of a display screen or portion thereof with a graphical user interface showing the new design; and,

FIG. 2 is a front view of the second embodiment thereof.

The broken lines showing the display screen or portion thereof form no part of the claimed design. The broken lines showing portions of the graphical user interface form no part of the claimed design. The broken lines showing environmental structure form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 